The invention relates to a holographic recording process using a thermoplastic photosensitive member. As is well recognized, holography represents a recording process in which both the intensity and phase of light are recorded to permit a three dimensional reproduction of an image of an object being recorded. Specifically, a light having a time and spatial coherence, for example, a laser light, is split into a pair of beams along different paths, one of which is employed as an object illuminating light and the other as a reference light so as to cause an interference of the latter with the object light, that is, the object illuminating light which is reflected by the object. The resulting interference fringe is recorded on a sensitive member, for example, a thermoplastic photosensitive member 2 which is located at a given position on an optical bench 1, as shown in FIG. 1. In this Figure, the reference character O represents an object being recorded, and LO and LR an object illuminating light and a reference light, respectively. The interference fringe recorded on the thermoplastic photosensitive member 2 is developed to provide a hologram, and the described process is referred to as a holographic recording.
As mentioned previously, both the intensity and phase of the object light LO are recorded on the hologram, and therefore if the thermoplastic photosensitive member 2 carrying the hologram is disposed at the same position as it has been placed during the recording, and then illuminated with a laser light of the same wavelength as the reference light LR, which is utilized as a reproducing light, in the same direction as the reference light LR has been directed, the reproducing light will be diffracted by the hologram to reproduce the wave front which existed on the surface of the member 2 during the recording process, thereby reproducing a three dimensional, virtual image at the position of the object O shown in FIG. 1.
The thermoplastic photosensitive member 2 comprises an image receptor material having both photoconductivity and thermoplasticity, one example of which is illustrated in FIG. 2. Specifically, the illustrated thermoplastic photosensitive member 2 comprises a lamination of a thermoplastic resin layer 2-1, which will be referred to as thermoplastic layer, a photoconductive layer 2-2 and a conductive layer 2-3 disposed in the sequence named on a base layer 2-4. The thermoplastic layer 2-1 is electrically insulating and has an electrical resistance comparable to that of the photoconductive layer 2-2 when placed in a dark place. The thermoplastic photosensitive member 2 which is used in forming a hologram is entirely constituted by a transparent material so that the reproduced virtual image can be observed through the hologram.
There are two broad categories of techniques for recording an image on a thermoplastic photosensitive member, namely, a sequential process and a simultaneous process. The invention is directed to the latter process, which will be specifically described below. As illustrated in FIG. 3, the surface of the thermoplastic layer 2-1 of the thermoplastic photosensitive member 2 is charged to the positive polarity, for example, and is exposed to a light image. In an illuminated area of the surface, the photoconductive layer 2-2 lying therebelow will become conductive to induce a negative charge in the interface between the thermoplastic layer 2-1 and the photoconductive layer 2-2 in such area, while in a dark area, the negative charge will be induced only in the interface between the conductive layer 2-3 and the photoconductive layer 2-2. As a result, the capacitance will be increased in a bright area as compared with a dark area. By charging the above surface to the positive polarity, the charge density in the bright area will be substantially increased relative to that in the dark area, thus forming an imagewise electrostatic latent image by a variation in the surface charge density.
Subsequently, a current is passed through the conductive layer 2-3 by connecting it with a d.c. source E, as shown in FIG. 4, while the electrostatic latent image remains in the thermoplastic photosensitive member 2, so as to produce Joule heat which will heat the photosensitive member 2 to a temperature close to the softening point of the thermoplastic layer 2-1. Since the electrostatic force acting in the direction of the thickness of this layer is proportional to the square of the surface charge density, the softened thermoplastic layer 2-1 will be deformed in accordance with the latent image. For example, an area having a high charge density and thus corresponding to a bright area of the light image will be recessed. In this manner, an image of unevenness corresponding to the latent image is formed in the surface of the thermoplastic photosensitive member 2. Immediately upon formation of such a deformation image, the thermoplastic photosensitive member 2 is cooled to a temperature below the softening point, whereby the thermoplastic layer 2-1 is solidified while maintaining the deformation image, which is therefore fixed in the surface thereof. This process is referred to as thermal developing.
To form a hologram on or in the thermoplastic photosensitive member 2 thus means a recording of an interference fringe of the object light and the reference light as an unevenness image in the surface of the thermoplastic layer 2-1. As a recording member, a thermoplastic photosensitive member has excellent features of high sensitivity and repeated useability, but it involves a number of problems for practical use because of the fact that noises are produced in the reproduced image or the quality of the reproduced image is degraded as it is repeatedly used.